There are three main types of candles used on altars:

    God and goddess candles are usually gold (god) and silver (goddess). They are meant to represent and honor the masculine and feminine energies. However, if you work with patron deities, you may wish to use candles which correspond to those deities specifically. Gold and silver are general colors that represent masculine and feminine energy, and so can be used when working with any god and goddess. The god candle is gold because the god is associated with the sun (warm and dry), and the goddess candle is silver because the goddess is associated with the moon (cool and damp). These two candles should be places near the center of the altar, with the god candle on the right half and the goddess candle on the left half, as shown below.

   The elemental candles represent the energies of the four elements. The colors and placements are usually as follows: green in the north (earth), yellow in the east (air), red in the south (fire), and blue in the west (water). These candles usually go near the edge of the altar, but make sure they won't fall or get knocked over the edge. See the image below for specific placement.

    The finished product, according the the above two diagrams, is as follows:

   Illuminator candles are simply for added light, if you need it, and placement, color, etc. isn't critical. Although the feeling of working in candlelight is magickal in itself, you may be reading or doing something else that does require more light than what's provided by the god/goddess and elemental candles. I use three simply because three is such a magickal number, and I situate them in the shape of a triangle in the center of the altar. If you don't need the extra light, don't use them unless you really want to, or you'll end up spending more money on candles than you need to.